In recent years, in the field of Graphic Arts, have been introduced direct-positive silver halide photographic elements for duplicating processes. Said elements are designed for exposure to UV light on high powered contact frames which are available for exposing lithographic plates and may be handled in ordinary room light (typically in bright white lighting, in which the term "white light" is defined as the emission of a typical commercially available fluorescent lamp).
It is very desirable to produce silver halide photographic elements for duplicating processes which may be handled safely in bright white light. The benefits of this include ease of working and inspection of the element during exposure and processing, and generally more pleasant working conditions for the operators.
While negative acting silver halide elements are resistant to fogging in room light by making use of an accentuated low intensity reciprocity failure effect, direct-positive silver halide elements do not show this effect to any great extent. For this reason direct positive silver halide elements for duplicating purposes cannot approach the degree of white light safety displayed by negative acting elements.
In the absence of a low intensity reciprocity failure effect, the white light handling characteristics can be effected by acting on the overall speed. Obviously, the slower the speed of a silver halide element with respect to the exposing light, the better its tolerance to room light.
Direct-positive silver halide elements which are intended for exposure to UV emitting lamps (such as metal halide lamps) may be afforded some degree of white light handleability by the use of filter dyes either in the emulsion layer or in a non light-sensitive layer. Typically, the dye will particularly absorb visible light and prevent exposure of the radiation sensitive layer by this light. The filter dye is chosen such as not to interfere in a high extent with intentional exposure to UV radiations. Examples of silver halide elements incorporating said filter dyes are disclosed in U.S. Pat. Nos. 4,140,531, 4,232,116 and 4,495,274 and EP Pat. application Ser. No. 146,302 where blue absorbing dyes are used which are bleachable during processing.
The improvement in white light handling which may be achieved by the use of filter dyes is not normally very great unless very high dye loadings are used. Such high dye loadings tend to cause problems such as a residual dye stain, increase in Dmin (fog) and decrease in contrast. Moreover, the use of filter dyes may chemically interfere with the reversal process and still further exposure beyond the minimum density can sometimes result in an increase of density or re-reversal.
Therefore there is a need in the duplicating films in the field of Graphic Arts to provide UV sensitive direct-positive silver halide photographic elements which can be handled under white light without any significant loss in image density.